How can I replace my old boiler timer WITHOUT thermostat?

I have an old Sangamo M5 boiler timer, ticking like mad! Wanting to replace it with a modern programmable timer and looks like I'll go for the Drayton Lifestyle LP522.

I have a gravity fed HW system. The CH system does not have a room thermostat connected anywhere. My father has said he thinks this is impossible as there is nothing to turn off the boiler in the case that all the radiator valves shut off??

In the instructions for the LP522, it shows how to wire the programmer with a room thermostat but what do I do if I don't have one? You can see the instructions here:

http://lp522guide.tripod.com/sitebuildercontent/sitebuilderfiles/lp522guide.pdf

Any help would be much appreciated, thanks.